This invention relates to a process and apparatus for achieving customer selectable colors in a development device, such as an electrostatographic imaging system. More particularly, this invention relates to a process and apparatus for achieving customer selectable colors and customer selectable highlight colors in an electrostatographic imaging device using a hybrid development system or a conductive magnetic brush development system and a developer composition comprised of a blend of two or more toners from a base set of compatible color toner compositions. These may be incorporated into either a conventional single or multi-pass electrostatographic system or a tri-level single pass highlight color electrophotographic system. This invention also relates to a process and apparatus wherein two or more toner compositions from the base set of compatible color toner compositions are mixed with carrier particles in the development apparatus, enabling on-line color selection and correction.
Generally, the process of electrostatographic printing includes the step of charging an imaging member to a substantially uniform potential to sensitize the surface thereof. The charged portion of the surface is exposed to an image, such as an image of an original document being reproduced, or to a computer-generated image written by a raster output scanner. This records an electrostatic latent image on the imaging member corresponding to the original document or computer generated image. The recorded latent image is then developed by bringing a developer material into contact therewith. In a tri-level system, three separate potential levels are used. The unexposed areas of the latent image are developed in one color, and the fully discharged areas are developed in another color; the partially exposed areas remain undeveloped. This forms a toner powder image on the imaging member that is subsequently transferred to a substrate, such as paper. Finally, the toner powder image is permanently affixed to the substrate in image configuration, for example by heating and/or pressing the toner powder image.
A suitable developer material may be a two-component mixture of carrier particles having toner particles triboelectrically adhered thereto. The toner particles are attracted to and adhere to the electrostatic latent image to form a toner powder image on the imaging member surface. Suitable single component developers are also known. Single component developers comprise only toner particles; the particles have an electrostatic charge (for example, a triboelectric charge) so that they will be attracted to, and adhere to, the latent image on the imaging member surface.
There are various known forms of development systems for bringing toner particles to a latent image on an imaging surface. One form includes a magnetic brush roll that picks up developer from a reservoir through magnetic attraction and carries the developer into proximity with the latent image. In a modification of the magnetic brush apparatus, known as hybrid development, the magnetic brush roll does not bring toner directly to the imaging member surface, but transfers toner to a donor roll that then carries the toner into proximity with the latent image. In single component scavengeless development, a donor roll is used with a plurality of electrode wires closely spaced from the donor roll in the development zone. An AC voltage is applied to the wires to form a toner cloud in the development zone and the electrostatic fields generated by the latent image attract toner from the cloud to develop the latent image. In a hybrid scavengeless development system, the method of development with a donor roll is the same as in single component scavengeless development, except that a magnetic brush is first used to load the donor roll with toner. In single component jumping development, an AC voltage is applied to the donor roll, causing toner to be detached from the roll and projected towards the imaging member surface. The toner is attracted by the electrostatic fields generated by the latent image and the latent image is developed. Variants of these development systems may be used with single component or two-component developers.
For example, U.S. Pat. No. 5,032,872 to Folkins et al. describes the use of a hybrid scavengeless development system having dual donor rolls and electrically biased electrodes for each donor roll. The hybrid development system uses a two-component magnetic brush to supply toner particles to one or more donor rolls, which in turn load the toner particles onto a wire in the development nip for developing the latent image. Although the described hybrid development system contains multiple donor rolls, the separate donor rolls are supplied with toner from a common developer reservoir by a common magnetic brush roll.
Various forms of systems for producing two-color developed images are also known. For example, U.S. Pat. No. 4,078,929 to Gundlach teaches the use of a tri-level electrostatographic system as a means to achieve singlepass highlight color images. Gundlach teaches a method for two-color development of an electrostatic charge pattern of a single polarity and having three different levels of potential by utilizing relatively negatively charged toner particles of one color and relatively positively charged toner particles of a second color. In this method, the photoreceptor is initially charged to a voltage V.sub.0. It is then selectively discharged with a single raster output scanner to approximately V.sub.0 /2 in the background areas and to near 0 or residual potential in the color areas. The fully discharged areas are printed in color, and the unexposed areas, which undergo dark discharge, are printed in black (or a second color). Alternatively, the colors may be reversed, i.e., the unexposed areas may be developed in color, and the areas of near 0 or residual potential may be developed in black (or a different color).
Another method of two-color reproduction is disclosed in U.S. Pat. No. 3,013,890 to Bixby. Bixby teaches a method in which a charge pattern of either a positive or negative polarity is developed by a single, two-color developer. The developer of Bixby comprises a single carrier that supports both triboelectrically relatively positive and relatively negative toner. The positive toner is a first color and the negative toner is a second color. The method of Bixby develops positively charged image areas with the negative toner and develops negatively charged image areas with the positive toner. A two-color image occurs only when the charge pattern includes both positive and negative polarities.
Toner compositions and methods for producing such compositions are known.
Several methods of producing blended color toners and developers are also known. Traditionally, the methods have focused on combining multiple colorants into a single toner composition, or using multiple separate developers each containing a single colored toner. For example, U.S. Pat. No. 4,312,932 to Hauser et al. discloses toners and developers wherein the developing composition comprises a single carrier and toner resin particles containing up to four pigments. Suitable pigments are described to be those selected from magenta, cyan, yellow and white. The process involves incorporating multiple appropriate pigments into the toner resin, for example, by blending the pigments together in the molten resin polymer during the processing and preparation of the toner resin, to yield a toner or developer with the desired specific color.
As a further example, U.S. Pat. No. 4,395,471 to Hauser et al. describes a process whereby three color toners are mixed in an appropriate ratio to yield a final toner composition of a desired specific color. In that process, specified amounts of previously-prepared yellow, cyan, and/or magenta toners are mixed together in their powdered or ground form and then combined with a single carrier to yield a developer with a specific color. The toners and process of the reference are disclosed as especially useful in flat color copying, which requires the use of halftoning if paler colors are to be produced. A similar process is also disclosed in U.S. Pat. No. 5,204,208 to Paine et al., which describes forming a customized color toner by mixing together specified amounts of at least two encapsulated toners of different colors.
A group of compatible toner and developer compositions, which allow for easier mixing of the separate color toners in various combinations, is disclosed in allowed U.S. Ser. No. 08/024,134 filed on Mar. 1, 1993. Disclosed are toner compositions that may be used to provide a palette, that is for example a set of pre-selected color toners or an extended set of colors, by admixing certain toner compositions. Thus, a small starting set of toners, such as red, green, blue, cyan, magenta and yellow, may be mixed or blended to generate many other colors by the method of co-mixing two or more of these toners, to provide toners with pre-selected colors. Each new co-mixture, with a relative ratio of the constituent toners, may become a new toner to be added to a carrier to form a developer composition particularly useful in tri-level or color electrophotographic processing. The toner compositions are disclosed as usually containing pigment particles comprised of, for example, carbon black, magnetite, cyan, magenta, yellow, blue, green, red or brown components, or mixtures thereof, thereby providing for the development and generation of black and/or colored images. The toner compositions possess excellent admix characteristics and maintain their triboelectric charging characteristics for an extended number of imaging cycles.
Highlight color and customer selectable color printing is a very difficult process. Due to the strict requirements of triboelectric charging, admix and fusing characteristics of the toner composition, it is often difficult to design even a single toner composition for a tri-level printing process. This is especially true as faster and more complex imaging and printing methods are being developed, as more copies or prints are being made by the systems, and as the demands made on the toner and developer compositions are ever increasing. These concerns are further heightened in processes of the present invention, where a blend of two or more base color toners is used to form a final color toner composition.
For example, in the case of a hybrid development system or a hybrid scavengeless development system, development of the electrostatic latent image essentially occurs twice. First, the blend of the base color toners is transferred to (developed uniformly on) a donor roll; and second the blend is transferred to (developed imagewise on) an imaging member to develop the latent image. In each step, the blend of the base color toners must be homogeneous, in order to ensure that the color of the final developed image is uniform.
In the case of a tri-level development system, the concerns for formulating each of the toners is further exacerbated by the fact that the triboelectric charging requirements are more strict. For example, in a conventional tri-level imaging system, each of the two toners being used must have a different triboelectric value, must be relatively oppositely charged with respect to an intermediate potential, and must not interact with each other or with other portions of the latent image. As such, the formulation of each of the two toners must account for such factors as how the triboelectric charge of the toner particles will be affected once the toner is charged into a developer housing with carrier particles. In the case of blends of toner compositions in one developer housing, it is further required that the constituent toners (i.e. the base color toners) also be formulated to account for the effects on the triboelectric charge of the individual toners as they are mixed together.
Furthermore, in all of the development systems, the toner formulations must also account for the ever-increasing demands being placed upon the process and the individual system components. For example, as more prints are being made over the same period of time, it is necessary that the toner and developer compositions exhibit higher stability than was previously required. Similarly, the toner and developer must exhibit improved admix characteristics in order to ensure uniform triboelectric charge as fresh toner or replenisher is added to the development system and to ensure a uniform blend of the developer materials. At the same time, higher print quality is being demanded from the toner compositions, including blended toner compositions.